This invention relates to axial split-pin tumbler-type lock mechanisms, particularly, to a lock mechanism having increased pick-resistance.
In general, the axial split-pin tumbler-type lock mechanisms include a rotatable operating part and a stationary part, which adjoin at an interfacial plane. Tumblers each including a driver element and a follower element are mounted in bores in the parts which meet in alignment at the interfacial plane, and the tumblers are movable back and forth in the aligned bores. When the interfacial plane is bridged by one or more of the tumbler elements, the operating and stationary parts thereby are secured against rotation relative to each other. When the joints between the tumbler elements coincide with the interfacial plane on insertion of the proper key, the operating part may be rotated by means of the key, to accomplish a desired function. Kerr U.S. Pat. No. 3,270,538 is illustrative of one form of lock mechanism to which the present invention is directed.
The lock mechanisms commonly are picked with what may be referred to as a "gang pick," of the type disclosed in the foregoing patent. The pick is applied to a lock mechanism, a rotational torque or bias is applied to the operating part by the pick, to displace the operating part to a slight extent relative to the stationary part, and a back and forth jiggling motion is utilized to catch tumbler elements on tumbler bore margins which project over previously aligned adjoining bores, until elements of all of the tumblers have been caught at the interfacial plane and the operating part is free to rotate, the mechanism then being in an unlocking condition.
The aforesaid patent discloses the provision of tumbler-biasing springs which vary among themselves in their spring rates or strengths. The differences in the spring rates are responsible for altering the manner in which the tumblers move when the lock pick is used, thereby interfering with the intended operation of the pick. While the sole provision of springs having different rates increases the pick-resistance of the lock mechanisms in this manner, certain techniques have, nevertheless, been found to enable a skilled operator to open the lock mechanisms with a pick of the type shown in the patent.